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168           MORPHOLOGY AND  EVOLUTION
Sterno- and Cleido-Mastoid (fig. 32).—These muscles are separate in the vast majority of examples of all the Apes. The former runs from the front or lower border of the manubrium sterni to the occipital crest, so is really a sterno-occipitalis. The cleido-mastoid runs from the inner third of the upper border of the clavicle to the mastoid area or process in the Gorilla, Chimpanzee and Orang, or to the wall of the external auditory meatus in the Gibbons. Occasionally there is a sterno-mastoid and cleido-occipital.
The omo-liyoid is a single undivided muscle in the Orang, Gibbons and some Chimpanzees, but it is a double muscle in Man, the Gorilla and most Chimpanzees. It is said to be occasionally absent in the Orang. And in one Chimpanzee (fig, 3lB) it had three bellies.
The digastric muscle* vfig. 32) varies considerably. In Man, the Gorilla and some Chimpanzees there are two anterior and two posterior bellies connected by an intermediate tendon, which is connected fco the hyoid bone by a fibrous arch. In other Chimpanzees the anterior bellies are fused as in the Macaques to form a broad muscle concealing the mylo-hyoid muscle. The anterior bellies are more powerful in Man than in the Apes, and Duckworth (50) points out that the size is inversely proportional to that of the mandible. Hence, with a lighter mandible, Man requires a stronger depressor. The anterior belly plays a more important part than the mylo-hyoid as a support for the tongue in
* See papers by Bovero (22), Ch£ine (317), Dobson (i6sAj, Futamura (64), Parsons (127X1, and Toldt (290).